In many situations, sophisticated authentication mechanisms are used to discern whether a user requesting access to a network resource is a human or an automated machine, often called a “bot.” For example, completely automated public turing tests to tell computers and humans apart (“CAPTCHA”) have been used to enhance security measures and prevent automated machines from accessing a particular web-resource. Typical image verification methods involve the display of a plurality of images of objects to the user and a request that the user select one or more of the images that contain objects with a particular characteristic. For instance, an image verification challenge may request that the user select or identify all images of the plurality of images that contain a specific type of object (food, animal, vegetation, etc.). However, some bot programmers have developed ways to answer image verification by identifying the correct images without the need for a user to participate, including using “random guessing” with high volume traffic or using metadata from image search results.